Communication system

ABSTRACT

The present application relates to a wireless communication system and related methods and apparatuses for transmitting a signal from a source apparatus to a destination apparatus, via at least one intermediate apparatus. In particular, the present invention relates to techniques which seek to improve the throughput of data in multi-hop communication systems.

The present invention relates to a wireless communication system and related methods for transmitting a signal from a source apparatus to a destination apparatus, via at least one intermediate apparatus. In particular, the present invention relates to techniques which seek to improve the throughput of data in multi-hop communication systems.

It is known that the occurrence of propagation loss, or “pathloss”, due to the scattering or absorption of a radio communication as it travels through space, causes the strength of a signal to diminish. Factors which influence the pathloss between a transmitter and a receiver include: transmitter antenna height, receiver antenna height, carrier frequency, clutter type (urban, sub-urban, rural), details of morphology such as height, density, separation, terrain type (hilly, flat). The pathloss L (dB) between a transmitter and a receiver can be modelled by: L=b+10n log d  (A)

Where d (metres) is the transmitter-receiver separation, b(db) and n are the pathloss parameters and the absolute pathloss is given by l=10^((L/10)).

FIG. 1A illustrates a single-cell two-hop wireless communication system comprising a base station (known in the context of 3G communication systems as “node-B” (NB)) a relay node (RN) and a user equipment (UE). In the case where signals are being transmitted on the downlink (DL) from a base station to a destination user equipment (UE) via the relay node (RN), the base station comprises the source apparatus (S) and the user equipment comprises the destination apparatus (D). In the case where communication signals are being transmitted on the uplink (UL) from user equipment (UE), via the relay node, to the base station, the user equipment comprises the source apparatus and the base station comprises the destination apparatus. The relay node is an example of an intermediate apparatus (I) and comprises: a receiver, operable to receive a signal from the source apparatus; and a transmitter, operable to transmit this signal, or a derivative thereof, to the destination apparatus.

Table I below gives some examples of the calculated pathloss of a signal being transmitted over the different links: source to destination (SD), source to intermediate (SI) and intermediate to destination (ID), in a multi-hop transmission system where b and n are assumed to remain the same over each of the links. TABLE I Separation Pathloss Absolute (metres) in dB Pathloss b(dB) n SD SI ID SD SI ID SD SI ID 15.3 3.76 1000 500 500 128.1 116.8 116.8 6.46E12 4.77E11 4.77E11 15.3 3.76 1000 600 600 128.1 119.76 119.76 6.46E12 9.46E11 9.46E11 15.3 3.76 1000 700 700 128.1 122.28 122.28 6.46E12 1.69E12 1.69E12

The examples calculated above demonstrate that the sum of the absolute path losses experienced over the indirect link SI+ID may be less than the pathloss experienced over the direct link SD. In other words it is possible for: L(SI)+L(ID)<L(SD)  (B)

Splitting a single transmission link into two shorter transmission segments therefore exploits the non-linear relationship between pathloss verses distance. From a simple theoretical analysis of the pathloss using equation (A), it can be appreciated that a reduction in the overall pathloss (and therefore an improvement, or gain, in signal strength and thus data throughput) should be achieved if a signal is sent from a source apparatus to a destination apparatus via an intermediate apparatus (eg relay node), rather than being sent directly from the source apparatus to the destination apparatus. If implemented, multi-hop communication systems could potentially allow for a reduction in the transmit power of transmitters which facilitate wireless transmissions, which would lead to a reduction in interference levels as well as decreasing exposure to electromagnetic emissions.

Clearly, due to the non-linear relationship between pathloss and distance, the position of an intermediate apparatus relative to the source and destination, will critically effect the potential gain that a multi-hop transmission may have as compared to a direct, or single-hop, transmission between the source and destination. This is illustrated in FIG. 2A which shows a graphical representation of the theoretical gain which may be achieved by multi-hop transmissions, and plots the total power loss (dB) against the relative normalised position of the intermediate apparatus between the source apparatus and the destination apparatus.

Considering firstly the case where the intermediate node is positioned on the line of the direct link between the source and destination (in which case the path extension factor (s)=1), it can be seen that the potential gain is reduced as the relay node is moved away from a mid-way position towards the source or destination apparatus. Likewise, as the position of the intermediate apparatus is moved away from the line of the direct link, thereby extending the total path length of the sum of the two transmission segments (and increasing the path extension factor to s=1.1, s=1.2 etc), it can be seen that the graphical region of theoretical gain is again reduced.

However, simulations carried out to test the applicability of multi-hop communication systems have revealed unexpectedly low gains in throughput of data. Indeed, the gains experienced are well below the potential gain suggested by a simple analysis based on the pathloss equation A. Consequently, and despite the potential advantages that multi-hop systems may demonstrate in terms of signal range extension, a possible reduction in the overall transmit power required to transmit a signal between a source and destination, and the connectivity of otherwise inaccessible nodes, wireless systems operators have been deterred from implementing multi-hop networks.

One of the reasons that such a discrepancy exists between the predicted gain and the simulated gain is that previous predictions have been based on the assumption that the pathloss parameters b and n are the same on all links. In actual fact, these values vary as a result of the antenna height of the source apparatus and destination apparatus as compared to the height of the relay node. Thus, a more realistic table of values is given below in table II. The values labelled 3GPP are obtained from adapting the model employed by the 3GPP to incorporate the fact that the antenna height of the intermediate apparatus is typically somewhere between the height of the antenna at the source and destination apparatus. The values labelled UoB are derived from modelling conducted by the University of Bristol based on a typical deployment in the city of Bristol. TABLE II Pathloss Link Parameter S-D S-I I-D 3GPP b (dB) 15.3 15.5 28 n 3.76 3.68 4 UoB b(dB) 13.07 16.29 10.04 n 4.88 4.64 5.47

The graphical illustration of total pathloss verses normalised relay node position using the pathloss parameters tabulated in table II is shown in FIG. 2B. It can be seen that the perfect “bell-shape” of FIG. 2A is not achieved when a more realistic set of pathloss parameters are used to calculate the variation in total pathloss as the position of a theoretical relay node is adjusted. Indeed, the region of gain is reduced and it is apparent that relatively small changes in the position of a relay node or a user equipment, leading to a change in the absolute pathloss over the communication link, will have a significant effect on the quality of a communication signal at the receiving apparatus. Thus, the positioning of an intermediate apparatus or relay node is critical if a gain is to be achieved by the occurrence of a multi-hop transmission, as compared to a direct transmission between the source and destination.

However, even when predictions are based on a more accurate reflection of the pathloss parameters likely to be encountered in the real world, simulations of multi-hop systems have revealed unexpectedly poor correspondence between the predicted and simulated gain.

Embodiments of the present invention seek to provide a communication system comprising a source apparatus, a destination apparatus and at least one intermediate apparatus, wherein the source apparatus and the or each intermediate apparatus each comprise a transmitter, operable to transmit a communication signal or a signal derived therefrom, in a communication direction towards said destination apparatus, and wherein the destination apparatus and the, or each, intermediate apparatus each comprise a receiver, operable to receive said communication signal, or a signal derived therefrom, wherein said communication system comprises a determining means, operable to determine a measure of, or a change in a measure of, the resource allocated to one or more of said transmitters that will tend to substantially attain or maintain a balance between:

i) a measure of the quality of the communication signal received at the destination apparatus; and

ii) measure of the quality of the communication signal received at the, or each, intermediate apparatus.

It will, of course, be appreciated that the communication signal actually received by the destination apparatus may be the communication signal transmitted by the source apparatus, or it may be a communication signal derived therefrom.

Thus, preferred embodiments of the present invention seek to maintain or achieve a “balance” in a measure of the quality of a communication signal being received at the or each intermediate apparatus and a measure of the quality of a communication signal being received at a destination apparatus. Preferably, the determining means is operable to determine a change in the transmit power of one or more of the apparatuses which are operable to transmit a communication signal present communication system embodying the present invention, in order to reduce or prevent substantial imbalance (i.e. achieve or maintain a substantial “balance”) between a measure of the quality of a communication signal received at the intermediate apparatus and a measure of the quality of a communication signal received at the destination apparatus.

The existence of an imbalance arising in a communication system embodying the present invention may be apparent from a direct comparison of a measure of a quality of a communication signal received at the destination apparatus and a measure of the quality of a communication signal received at the, or one of the, intermediate apparatuses. Alternatively, an imbalance may be apparent when a comparison is made via a mapping function. Hence the situation may exist where measures of equal value do not equate to a balanced system, and likewise where measures of differing value may equate to a balanced system.

It is envisaged that embodiments of the present invention may be used, prior to deployment of a multi-hop system, to optimise the system and/or to substantially balance a measure of the quality of a communication signal received at the, or each intermediate apparatus and a measure of the quality of a communication signal received at the destination apparatus. It is also envisaged that embodiments of the present invention may be implemented within an existing multi-hop system in order to seek to achieve and maintain “balance” in a measure of the quality of a communication signal across all links. Thus, the present invention may be employed within a multi-hop communication system to establish a substantial “balance” between an indicator of the RSS or the SINR at the destination apparatus and an indicator of the RSS or the SINR, at the, or each, intermediate apparatus. The transmit powers will advantageously be optimised initially with respect to a target received signal quality for one of the apparatuses operable to receive a communication signal in a multi-hop system. This will usually be the destination apparatus. Thus, an indicator of a measure of the variation of the quality of a communication signal received at the destination from a target received signal quality (=“variation from target” indicator), will advantageously be minimal when a system has been optimised according to embodiments of the present invention. Thereafter, if a change is detected in the variation from target indicator, which may be in a positive or negative sense, e.g. if the quality of the communication signal has deteriorated or improved, or if the target set for the apparatus has changed, the variation from target indicator will increase. In this case, embodiments of the present invention which enable a deviation of the variation from target indicator from a desired value to be detected, will advantageously seek to bring the variation from target indicator to the desired value.

Simulations of multi-hop communication systems embodying the present invention have been found to demonstrate a significant gain over systems in which a signal is transmitted directly to a destination apparatus. Indeed, the results of system level simulations carried out to test a preferred embodiment of the present invention indicate that a communication system which is “balanced” within the context of the present invention, can be expected to fulfil the advantages associated with multi-hop transmissions and to provide an improvement in the throughput of data.

It is believed that one explanation for the improved throughput demonstrated by preferred embodiments of the present invention is that they permit a reduction in the absolute transmit power required in a multi-hop system. This is considered in more detail below.

Starting from the principle already demonstrated above, that by splitting a single direct transmission link into two shorter transmission links, a reduction in the total pathloss experienced by a signal is achieved. Then, the total transmit power required to transmit a communication signal from a source apparatus to a destination apparatus via at least one intermediate apparatus, will be less than is required to transmit the communication signal directly between the source apparatus and the destination apparatus. Thus, less transmit power is needed in order to ensure that the destination apparatus (and possibly also the intermediate apparatus) receives a minimum or “target” signal quality. If no adjustment is made to the transmit power, then significant excess transmit power (i.e. transmit power exceeding that required to achieve a good, or target, signal quality at the destination apparatus and/or the intermediate apparatus) will result. Rather than serving to further increase the gain achieved by a multi-hop communication as compared to a direct communication between a source apparatus and a destination apparatus, this excess transmit power will merely increase interference levels leading to a deterioration in the quality of the communication link. This deterioration will tend to counteract the potential gain of a multi-hop system which accounts for the poor simulation results of previously considered multi-hop communication systems.

Furthermore, the overall throughput across a two-hop network (for example) is limited by the lower of: the number of data packets received at the intermediate apparatus and the number of data packets received at the destination apparatus. The number of data packets received at a receiver is dependent upon the quality of the communication link that terminates at that receiver. This may be reflected, for example, by a measure of the throughput, a measure of the received signal strength (RSS) or a measure of the signal-to-interference plus noise ratio (SINR). Thus, in effect, the receiver which receives the lowest quality communication signal within a multi-hop system forms a “bottle neck” for data packet transmission, thereby wasting capacity for data transfer on other links within the multi-hop system. An increase the transmit power at a transmitter which does not serve to improve the lowest quality communication signal, will result in additional excess transmit power. Consequently, a further degradation is experienced in the performance of the system. This is illustrated in FIGS. 9A and 9B which plot the variation of the gain in average packet throughput observed by users of a two-hop system compared to that observed for a single hop system, against the transmit power of the source apparatus (NB). Each graph includes four different plots, each representing a different transmit power of the intermediate apparatus. It can be seen that as the transmit power of the base station is increased beyond an optimal point, then a significant degradation in gain will be experienced despite the emission of more signal energy.

It can therefore be appreciated that the improvements made by preferred embodiments of the present invention can be attributed to the way in which the various aspects of the present invention seek to ensure that any imbalance between a measure of the quality of a communication signal received at the destination apparatus and a measure of the quality of a communication signal received at the, or each, intermediate apparatus is reduced or prevented. Thus, excess transmit power which cannot improve the throughput of data packets and which will only serve to raise interference levels, is minimised.

There are a number of different events which, if they arise, can potentially lead to an “imbalance” (i.e. a difference between a measure of the quality of a communication signal received at the destination apparatus and a measure of the quality of a communication signal received at the or each intermediate apparatus) in a multi-hop system:

-   -   i) The pathloss arising over one of the links changes. This may         be due to the position of one or both of the transmitter and         receiver for that link changing, or due to a change in the         environmental conditions or interference levels arising between         the transmitter and the receiver.     -   ii) It is usual for an apparatus which is operable to receive a         communication signal, to have a target RSS or target SINR. This         is usually set by the network provider and may vary depending on         the characteristics of the communication system or receiving         apparatus, or depending on the type of data to be transmitted.         The target RSS/SINR of a mobile phone or other user equipment         may vary and any change in target can be accommodated for by         adjusting the transmit power of the transmitting apparatus in         such a way as to tend to minimise a measure of the variation of         the quality of a communication signal received at the         destination apparatus from a target received signal quality         (i.e. “variation from target”). In the case of a multi-hop         system, merely adjusting the transmit power of one apparatus in         order to accommodate a change in target of one of the receiving         apparatuses, will lead to an imbalance within the system.

Embodiments of the present invention seek to provide a way of responding to an imbalance, or a potential imbalance, which arises as a result of each of these possible events in order to improve the throughput of data being transmitted on the uplink (UL) from a source apparatus to a base station via one or more intermediate apparatuses. In a standard communications system the uplink is the link between the UE and the NB. In the multi-hop case the UL refers to the link in which communication is directed towards the NB (e.g. UE to RN, RN to RN in the direction of NB and RN to NB). Furthermore, embodiments of the present invention seek to provide a way of optimising a multi-hop system whereby any target quality set by one or more of the receivers is substantially attained and the throughput of data across each link is substantially equal.

According to a first aspect of the present invention there is provided a communication system comprising a source apparatus, an intermediate apparatus and a base station, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the base station comprising indicator derivation means operable to derive one or more indicators of the quality of a communication signal received at the base station, and wherein the communication system further comprises:

-   -   i) control means provided in the base station;     -   ii) indicator deviation detection means operable to detect a         change in one said indicator derived by the base station from a         desired value;     -   iii) determining means operable, following the detection of such         a change, to determine a required change in the transmit power         of the intermediate apparatus that will tend to bring said         indicator to said desired value, wherein said determining means         further comprises request transmitting means operable to         transmit a request for a change in the transmit power of the         intermediate apparatus to the control means.

Embodiments of the first aspect of the present invention advantageously provide a way of responding to a deviation in the indicators derived by the base station from a desired value which may be due to i) a change in pathloss between the intermediate apparatus and the base station; or ii) a change in the target of the base station by calculating a new transmit power for the intermediate apparatus and the source apparatus. Advantageously, the change in the transmit power that is required will be relative to the degree of deviation detected by the indicator deviation detection means.

In accordance with an embodiment of the first aspect of the present invention, one of the indicators derived by said base station may comprises a measure of the strength of a communication signal received at the base station (e.g. RSS). Preferably however, one of the indicators derived by the base station comprises a measure of the signal-to-interference plus noise ratio (SINR) of a communication signal received at the base station, and/or it may comprise a measure of the variation of the quality of a communication signal received at the base station from a target received signal quality set for the base station. An indicator of the variation from target may be a variation from target RSS, a variation from target SINR or a variation from a target which is based on a combination of RSS and SINR. If a variation from target indicator derived by the base station changes, embodiments of the first aspect of the present invention will seek to bring the variation from target indicator to the desired value.

Preferably the control means is operable, following receipt of a request for a change in the transmit power of the intermediate apparatus, to issue a command, to said intermediate apparatus, commanding a change the transmit power of the intermediate apparatus. The control means may advantageously comprises input signal receiving means, operable to receive an input signal which allows the control means to determine if an increase in transmit power of the intermediate apparatus is prohibited. Therefore, if the required change in transmit power of the intermediate apparatus comprises an increase in transmit power, and following a determination by the control means that an increase in transmit power of the intermediate apparatus is prohibited, the control means is operable to ignore said request. However, if the required change in transmit power of the intermediate apparatus comprises an increase in transmit power, and following a determination by the control means that an increase in transmit power of the intermediate apparatus is not prohibited, the control means is operable issue a command to the intermediate apparatus, commanding a change in transmit power of the intermediate apparatus. The intermediate apparatus preferably comprises command receiving means operable to receive such a command from the control means of the base station. According to a preferred embodiment said intermediate apparatus is operable to determine, based on a maximum transmit power of the intermediate apparatus, if the intermediate apparatus can carry out the change in transmit power according to the command. Then, if the intermediate apparatus determines that it cannot carry out the change in transmit power according to said command, said intermediate apparatus is operable to determine a revised change in transmit power of the intermediate apparatus which can be carried out by said intermediate apparatus. The intermediate apparatus is operable to cause the transmit power of the intermediate apparatus to change in accordance with said request, or said revised request, as the case may be.

According to a second aspect of the present invention there is provided a communication system comprising a source apparatus, an intermediate apparatus and a base station, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the wherein each of the base station and the intermediate apparatus comprise: indicator derivation means operable to derive one or more indicators of the quality of a communication signal received at the base station or the intermediate apparatus respectively, the communication system further comprising:

-   -   i) imbalance detection means operable to detect an imbalance         between one said indicator derived by the base station and one         said indicator derived intermediate apparatus; and     -   ii) determining means operable, following detection of such an         imbalance by said imbalance detection means, to determine a         required change in the transmit power of the source apparatus         that will tend to reduce such an imbalance; and     -   iii) control means provided in said base station and operable,         following determination of said change, to issue a command to         said source apparatus commanding a change in the transmit power         of the source apparatus.

Embodiments of the second aspect of the present invention advantageously provide a way of adjusting the transmit power of the source apparatus in order to substantially restore a balance between a measure of a quality of a communication signal received at the base station and a measure of the quality of a communication signal received at the intermediate apparatus. The imbalance may be due to a change in pathloss between the source apparatus and the intermediate apparatus. Alternatively an imbalance may arise following operation by a communication system embodying the first aspect of the present invention to respond to a change in the target quality indicator of the base station, since in restoring the variation from target indicator to its original measure (by changing the transmit power of the intermediate apparatus), the quality indictors of the intermediate apparatus and the base station apparatus will no longer be balanced.

According to embodiments of the second aspect of the present invention, one said indicator derived by each of the intermediate apparatus and the base station comprises a measure of the strength of a communication signal received at the base station or the intermediate apparatus respectively (eg RSS). Preferably however, one said indicator derived by each of said intermediate apparatus and said base station comprises a measure of the signal-to-interference plus noise ratio (SINR) of a communication signal received at the base station or the intermediate apparatus respectively.

Preferably the source apparatus is operable, following receipt of a command, and wherein said command is for an increase in transmit power, to determine if it can carry out the command based on the maximum transmit power of the source apparatus. If said source apparatus determines that it cannot carry out the said command, the source apparatus is operable to determine a revised change in transmit power that will tend to reduce the said imbalance, and to carry out said revised change. Furthermore, the control means is preferably operable, following issuance of a command to the source apparatus and wherein said command was for an increase in transmit power, to monitor the indicator derived by the intermediate apparatus in order to determine if said transmit power of said source apparatus has been changed in accordance with said command. If it is determined that a change in transmit power of the source apparatus has not been carried out in accordance with said request, the control means is operable to prohibit any subsequent increases in transmit power of said intermediate apparatus. If an increase in transmit power of said intermediate apparatus is prohibited, and if no subsequent imbalance is detected by said imbalance detection means, said control means is then operable to allow a subsequent increase in transmit power of said intermediate apparatus. If an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detection means such that said control means of said base station is caused to issue a command to said source apparatus and wherein said command is for a decrease in transmit power, said control means is operable to allow a subsequent increase in transmit power of said intermediate apparatus. Furthermore, if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detection means such that said control means of said base station is caused to issue a command to said source apparatus and wherein said instruction is for an increase in transmit power which can be carried out by said source apparatus, said control means is operable to allow a subsequent increase in transmit power of said intermediate apparatus.

The intermediate apparatus preferably comprises a receiver operable to receive the signal transmitted by the source apparatus; and a transmitter operable to transmit the received signal, or a signal derived therefrom, to the destination apparatus. Duplexing of signals to separate communication signals received by the intermediate apparatus from communication signals transmitted by the intermediate apparatus may be Frequency Division Duplex (FDD) or Time Division Duplex (TDD). One or more of the intermediate apparatuses may preferably comprise a so-called relay node (RN) or relay-station (RS). A relay node has the capability of receiving a signal for which it is not the intended final destination and then transmitting the signal on to another node such that it progress towards the intended destination. A relay node may be of the regenerative type, where the received signal is decoded to the bit level, making a hard decision. If the received packet is found to be in error then retransmission is requested, hence the RN incorporates ARQ or H-ARQ. QRQ or H-ARQ is a receiver technique for managing retransmission request and subsequent reception of retransmitted signals. Once the packet is successfully received, it is then scheduled for retransmission towards the destination, based on any radio resource management strategies incorporated into the RN. Alternatively a relay node may be of the non-regenerative type, whereby data is amplified at the relay node and the signal is forwarded to the next station. It is envisaged that the function of an intermediate apparatus or relay node may be provided by a mobile phone, or other user equipment.

In accordance with embodiments of the first and second aspect of the present invention, which seek to adjust the quality indicators at the intermediate apparatus and the destination apparatus by adjusting the transmit power of the base station and the intermediate apparatus respectively and without explicitly calculating the transmit powers of the base station and intermediate apparatus, a relay node of the regenerative type is preferably used where the received signal is decoded to the bit level, making a hard decision. A regenerative relay node is operable to receive a communication signal transmitted by the source apparatus and to decode the signal to bit level before transmitting a new signal to the next station in the multi-hop system (which may be the destination UE or another intermediate apparatus).

The desired aim of the present invention is to set the allocation of resource at each transmitting apparatus such that the throughput across each link in the multi-hop system is equal. As the throughput is a function of the received SINR it follows that in order to balance the throughput across the multi-hop links, the received SINR at each node must be balanced. In the case of regenerative relays the SINR at a given apparatus is not a function of the SINR at any other node. This of course assumes equal SINR performance at all nodes. Thus, it is possible to ensure the required SINR which ensures that the system is substantially balanced and that the target SINR at the destination is met, can be achieved by simply adjusting the transmit power relative to the difference between the actual and required SINR. Further if the target SINR changes at one apparatus, then it is possible to adjust the transmit power at all nodes in a manner relative to the required change. Consequently, there is no need to calculate the actual transmit power and the implementation of embodiments of the present invention is advantageously computationally simple. It is envisaged that the function of an intermediate apparatus or relay node may be provided by a mobile phone, or other user equipment.

Whilst embodiments of the present invention may only realistically operate where regenerative relays are employed as the intermediate apparatus, they benefit from a relatively simple determination of the transmit powers which does not require the performance of an explicit calculation. Transmit powers are advantageously determined by adjusting the transmit power of the relevant transmitter relative to the degree of indicator change detected by the indicator deviation detection means, in order to restore the indicator which has experienced a change to its value, and thereby balance the received SINR.

Furthermore, embodiments of the present invention advantageously enable centralised control of the setting of the transmit power to be maintained, with minimal processing required in the relay station. This is beneficial to the operator of the wireless system as it keep control located within a central entity making management of the network much simpler. Further, should the relay start to malfunction, then due to the fact that control is located in the base station (or Node—B) then corrective measures are possible by the operator. Moreover, the fact that processing in the intermediate apparatus is kept to a minimum is advantageous in terms of reducing power consumption and thus maximising battery life, should the intermediate apparatus be a mobile or remote device.

The first and second aspects of the present invention will each tend to reduce or prevent an imbalance which arises or may arise, as the case may be, under different circumstances. For example, the situation may arise where the pathloss between the intermediate apparatus and the base station changes or the target of the base station may change. Both of these events leads to a change in the indicator derived by the base station and can advantageously be addressed by embodiments of the first aspect of the present invention. Preferably, a communication system embodying the first aspect of the present invention will comprise a deviation detection means which monitors the, or one of the, indicators of the destination apparatus at all times. Thus, any change or deviation in the indicator derived by the destination apparatus, can be detected quickly.

The first aspect alone may be sufficient to restore balance across a multi-hop system following a change in the pathloss between the intermediate apparatus and the base station. However, as discussed above, if the pathloss between the source apparatus and the intermediate apparatus changes (which may be due to a change in the position of the intermediate apparatus and/or the source apparatus or due to a change in the environmental conditions arising between the intermediate apparatus and the source apparatus), this must be dealt with by embodiments of the second aspect of the present invention. Moreover, in order to restore balance to a multi-hop communication system following a change in the target quality set by the destination apparatus, it is necessary for both the transmit power of the intermediate apparatus and the source apparatus to be adjusted. Thus, in order to deal with a change in the target quality indicator of the destination apparatus, a communication system which embodies both the first and second aspect of the present invention is preferably provided. Preferably, the imbalance detection of the second aspect of the present invention is performed periodically. Thus according to a preferred embodiment of the first aspect of the present invention said intermediate apparatus comprises indicator derivation means operable to derive an indicator of the quality of a communication signal received at the intermediate apparatus and wherein said intermediate apparatus is further operable to transmit said indicator to an indicator receiving means of said base station, wherein said base station further comprises an imbalance detection means operable to detect an imbalance between an indicator derived by the base station and an indicator derived by the intermediate apparatus, and wherein said determining means is further operable, following detection of such an imbalance by said imbalance detection means, to determine a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance, wherein said control means is further operable, following determination of said change, to issue a command to said source apparatus command a change in the transmit power of the source apparatus in accordance said required change.

The source apparatus is preferably operable, following receipt of a command, and wherein said command is for an increase in transmit power, to determine if it can carry out the command based on the maximum transmit power of the source apparatus. If said source apparatus determines that it cannot carry out the said request, the source apparatus is operable to determine a revised change in transmit power that will tend to reduce the said imbalance, and to carry out said revised change.

Preferably the control means is operable, following issuance of a command to the source apparatus and wherein said command was for an increase in transmit power, to monitor said indicator derived by the intermediate apparatus in order to determine if said transmit power of said source apparatus has been changed in accordance with said command. If it is determined that a change in transmit power of the source apparatus has not been carried out in accordance with said command, said control means is operable to prohibit any subsequent increases in transmit power of said intermediate apparatus. If an increase in transmit power of said intermediate apparatus is prohibited, and if no subsequent imbalance is detected by the imbalance detection means, said control means is operable to allow a subsequent increase in transmit power of the intermediate apparatus.

Preferably, if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detection means such that said control means of said base station is caused to issue a command to the source apparatus and wherein the command is for a decrease in transmit power, the control means is operable to allow a subsequent increase in transmit power of said intermediate apparatus. If an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by the imbalance detection means such that the control means of the base station is caused to issue a command request to the source apparatus and wherein the command is for an increase in transmit power which can be carried out by said source apparatus, said control means is operable to allow a subsequent increase in transmit power of said intermediate apparatus.

The situation may arise where a change in the target of the base station apparatus is accommodated by a substantially simultaneous change in the pathloss between the intermediate apparatus and the base station. Thus, in this case no request for a change in transmit power of the intermediate apparatus will be generated by the base station. This, relatively rare, situation can be handled by a communication system which embodies the second aspect of the present invention since a change in the measure of the pathloss experienced between the intermediate apparatus and the base station will give rise to an imbalance between the signal quality indicators derived by the intermediate and destination apparatuses and this imbalance will be detected by the imbalance detection means. The determining means is then operable to determine the change in the transmit power of the base station that is required to in order to tend to reduce the imbalance between a measure of a quality of a communication signal received at the intermediate apparatus and a measure of the quality of a communication signal received at the base station.

According to an embodiment of the first aspect of the present invention there is provided a method of controlling the transmit power of an intermediate apparatus in a multi-hop communication system, the communication system comprising a source apparatus, a base station and at least one intermediate apparatus, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the method comprising the steps of:

-   -   i) deriving, at the base station, one or more indicators of a         quality of a communication signal received at the base station;     -   ii) detecting a deviation in the, or one of the, indicators         derived by the base station from a desired value; and     -   iii) determining a required change in the transmit power of the         intermediate apparatus that will tend to bring said indicator to         said desired value.

According to an embodiment of the second aspect of the present invention there is provided a method of controlling the transmit power of a source apparatus in a multi-hop communication system, the multi-hop communication system comprising a source apparatus, a base station and at least one intermediate apparatus, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the method comprising the steps of:

-   -   i) deriving, at each of the base station and the intermediate         apparatus, one or more indicator(s) of a quality of a         communication signal received at the base station, or at the         intermediate apparatus, respectively;     -   ii) detecting an imbalance between one said indicator derived by         the base station and one said indicator derived by the         intermediate apparatus;

iii) determining a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance; and

-   -   iv) issuing a command to said source apparatus commanding a         change in the transmit power of said source apparatus.

According to another embodiment of the first aspect of the present invention there is provided a base station operable to receive, via one or more intermediate apparatus a communication signal from a source apparatus, the base station comprising:

-   -   i) indicator derivation means operable to derive one or more         indicators of a quality of a communication signal received at         the base station;     -   ii) indicator deviation detection means, operable to detect a         deviation in the, or one of the, indicators derived by the         indicator derivation means; from a desired value;     -   iii) determining means operable, following detection of such a         change by said indicator deviation detection means, to determine         a required change in the transmit power of the intermediate         apparatus that will tend to bring the said indicator towards         said first determining means being operable to transmit a         request for a change in the transmit power of the intermediate         apparatus to said control means;     -   iv) control means, operable to receive such a request from said         determining means.

The control means of the base station may advantageously further comprise input signal receiving means, operable to receive an input signal which allows the control means to determine if an increase in transmit power of the intermediate apparatus is prohibited.

Preferably, the base station may further comprise:

-   -   i) receiving means, operable to receive an indicator derived by         said intermediate apparatus, which indicator is indicative of a         quality of a communication signal received at the intermediate         apparatus;     -   ii) imbalance detection means operable to detect an imbalance         between an indicator derived by the base station and an         indicator received from the intermediate apparatus;     -   wherein said determining means is operable, following detection         of such an imbalance by said imbalance detection means, to         determine a required change in the transmit power of the source         apparatus that will tend to reduce such an imbalance, said         control means being further operable, following determination of         said required change, to issue a command to said source         apparatus commanding a change in the transmit power of said         source apparatus.

According to another embodiment of the second aspect of the present invention there is provided a base station operable to receive, via one or more intermediate apparatus, a communication signal from a source apparatus, the base station comprising:

-   -   i) indicator derivation means operable to derive one or more         indicator of the quality of a communication signal received at         the base station;     -   ii) indicator receiving means operable to receive one or more         indicator from the intermediate apparatus, wherein the or each         indicator is indicative of the quality of a communication signal         received at the intermediate apparatus;     -   iii) imbalance detection means operable to detect an imbalance         between an indicator derived by the base station and an         indicator received from the intermediate apparatus; and     -   iv) determining means operable, following detection of such an         imbalance by said imbalance detection means, to determine a         required change in the transmit power of the source apparatus         that will tend to reduce such an imbalance; and     -   v) said control operable, following determination of said         change, to issue a command to said source apparatus commanding a         change in the transmit power of said source apparatus.

Communication methods carried out in a base station embodying the present invention, an intermediate apparatus embodying the present invention or in a destination apparatus embodying the present invention are also provided.

Embodiments of the present invention are particularly suited to structured multi-hop systems which employ regenerative relays with either TDD or FDD duplexing to separate communication signals received at the intermediate apparatus from signals transmitted from the intermediate apparatus.

The desired value may be the value of the indicator of the quality of a communication signal derived by the destination apparatus which is at, or close to, the target value set by the destination apparatus, and when the system is substantially balanced (i.e. a measure of a quality of a communication signal received at the destination apparatus is in balance with a measure of a quality of communication signal received at the, or each, intermediate apparatus). Thus, embodiments of the first aspect of the present invention may be advantageously used to maintain the quality of the communication signal received by the destination apparatus at, or near, the target value set by the destination apparatus. Thereafter, it may be necessary for embodiments of a second aspect of the present invention to optimise the systems ensuring a balance is achieved between the destination apparatus and the or each intermediate apparatus.

Thus, it should be appreciated that the indication deviation detection means may be used in a system which has already been balanced, or optimised. Thus, a deviation from the desired value, which may arise due to an event which results in a change in a measure of a quality of a communication signal at the destination apparatus will be detected, and the required change the resource allocated to the previous intermediate apparatus determined.

If the indicator deviation is due to a change in the pathloss such that the quality of the communication signal received by the destination deviates from target, embodiments of the first aspect will advantageously restore balance to the system by adjusting the transmit power of the preceding intermediate apparatus. However, if the indicator deviation is due to a change in the target quality set by the destination apparatus, whilst embodiments of the first aspect may be advantageously employed to adjust the transmit power at the intermediate apparatus in order that the new target can be attained, embodiments of the second aspect are needed in order to restore a balance by determining the corresponding change in transmit power for the other transmitters in the multi-hop system.

Embodiments of the present invention may be implemented within a wireless communication system employing any multiple access technique, including but not limited to: frequency division multiple access (FDMA), time division multiple access (TDMA) code division multiple access (CDMA) and orthogonal frequency division multiple access (OFDMA). In the case of a CDMA system, in which all transmissions occur in the same frequency band and each transmission is assigned a unique channelisation code, the Gp factor represents the spreading factor or length of the code used to spread the transmitted signal otherwise known as the processing gain. In the case of orthogonal spreading codes, up to Gp channels are available for simultaneous transmission.

It should be appreciated that the term “user equipment” encompasses any device which is operable for use in a wireless communication system. Furthermore, although the present invention has been described primarily with reference to terminology employed in presently known technology, it is intended that the embodiments of the present invention may be advantageously applied in any wireless communication systems which facilitates the transmission of a communication signal between a source and destination, via an intermediate apparatus.

In any of the above aspects, the various features may be implemented in hardware, or as software modules running on one or more processors or a combination of these. The invention also provides operator programs (computer programs and computer program products) for carrying out any of the methods described herein, and computer readable media having stored thereon programs for carrying out any of the methods described herein. A program embodying the invention may be stored on a computer-readable medium, or it could, for example, be in the form of a signal such as a downloadable data signal provided from an Internet web site, or it could be in any other form.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

FIG. 1A illustrates a single cell/relay model of a wireless communication system;

FIG. 1B illustrates a two cell/relay model of a wireless communication system;

FIGS. 2A and 2B each show a graphical representation of the theoretical gain that may be achieved by a multi-hop communication system based on pathloss equation (A);

FIG. 3 illustrates an algorithm embodying the first aspect of the present invention;

FIG. 4 illustrates an algorithm embodying the second aspect of the present invention;

FIG. 5 illustrates parts of a base station embodying both the first and the second aspects of the present invention;

FIG. 6 illustrates the relationship between source transmit power and intermediate transmit power in the case of a multi-hop communication system having a non-regenerative relay node and using an FDD duplexing technique;

FIG. 7 illustrates the relationship between source transmit power and intermediate transmit power in the case of a multi-hop communication system having a non-regenerative relay node and using a TDD duplexing technique;

FIGS. 8A and 8B illustrate the optimal NB transmit power as a function of RN transmit power;

FIG. 9 shows a graphical illustration of the variation in the average gain in throughput observed by users of a multi-hop system as compared to that observed for a single hop system; and

FIG. 10 illustrate the optimal NB transmit power as a function of RN transmit power where it is assumed that the communication link between the source and destination apparatus has a 3 dB gain compared with the shorter multi-hop links.

An example of an algorithm which implements an embodiment of the first aspect of the present invention will now be described with reference to FIG. 3 in which the source apparatus comprises a user equipment (UE), the intermediate apparatus comprises a relay node (RN) which is of the type, and the destination apparatus comprises a base station (NB). The base station continually monitors the SINR and derives indicators of the SINR and the variation from target SINR.

The following sequence takes place following detection of a change in an indicator derived by the base station from a desired value in order to determine a change in the transmit power of the intermediate apparatus which will tend bring the indicator derived by the destination apparatus back to said desired value.

The details of the algorithm are summarised as follows: Uplink Algorithm 3: Part 1 Trigger: Periodically executed in NB Algorithm Input Required by Origin Request for change RN Change derived in NB and in RN Transmit Power processed by RN Block in increase RN Block is set/cleared in part 2 of the in RN Transmit Power algorithm Destination & Signalling Algorithm Output Derivation Requirement Change in RN transmit Relative change Relative change derived power at NB and made by RN

1. The base station detects a change in an indicator of SINR or in an indicator of the variation from target SINR such that the SINR at the destination apparatus does not meet its target.

2. The determining means of the destination determines the required change in the transmit power of the intermediate apparatus (RN).

3. A request is transmitted locally to a control means of the destination apparatus for a change in the RN transmit power.

4. If the request is for a decrease in the RN transmit power, the control means issues a command to the intermediate apparatus for a decrease in the transmit power of the RN.

5. If the request is for an increase in the RN transmit power, the control means checks whether a prohibition, or block, is currently in place which prohibits increases in the RN transmit power. Then:

5a. If it is determined that a prohibition is in place, the control means ignores the request; or

5b. If it is determined that no prohibition is in place, the control means issues a command to the intermediate apparatus; commanding an increase in the transmit power of the RN.

6. The RN receives a command from the control means of the NB and checks whether it can change its transmit power in accordance with the command. Then:

6a: If the RN determines that it cannot change its transmit power in accordance with the command, it determines a revised change in transmit power and adjusts its transmit power in accordance with the revised transmit power; or

6b: If the RN determines that it can change its transmit power in accordance with the command, the RN changes its transmit power accordingly.

The algorithm described above will manage the case of the propagation loss varying between the RN and NB and the case of the NB modifying its target RSS or SINR. In order to handle the case of the propagation loss varying between the UE and RN and the case that both the target in the NB and the propagation loss between the RN and NB varies, such that no request for change in RN transmit power is generated, an algorithm which implements an embodiment of the second aspect of the present invention operates periodically as discussed below.

The details of the algorithm are summarised as follows: Uplink Algorithm 3: Part 2 Trigger: Periodically executed in NB Algorithm input Required by Origin SINR at NB NB Known at NB SINR at RN NB Signalled from RN Destination & Signalling Algorithm Output Derivation Requirement Change in UE transmit power Relative Signalled to UE via RN change Block on RN power increase True/false test Part 1 of the algorithm

This algorithm is executed periodically in addition to the algorithm discussed above with reference to FIG. 4. Alternatively, it is also possible for this algorithm to be implemented separately in a wireless multi-hop communication system.

The algorithm assumes that indicators of the SINR at the NB and RN are reported to the NB.

1. The NB monitors the indicators of the SINR from both the NB and RN. Then:

1a. if these are found to vary such that they are imbalanced, a control means of the NB determines the change in transmit power of the UE that is required to restore a balance in SINR; or

1 b. If these are found to be balanced, a control means of the NB lifts any existing prohibition on an increase in the transmit power of the RN.

2. The control means issues a command to the UE, via the intermediate apparatus, commanding a change in the transmit power of the UE.

3. The UE receives the command from the NB and determines if it can carry out the required change in transmit power. Then:

3a. If it is determined that the UE cannot carry out the required change, the UE determines a revised change in transmit power and changes its transmit power in accordance with this revised change; or

3b. If it is determined that the UE can carry out the required change, the UE changes its transmit power in accordance with the required change.

4. If the command issued by the control means was for a decrease in the transmit power of the source apparatus, the control means lifts any existing prohibition on an increase in the transmit power of the RN.

5. If the command issued by the control means was for an increase in transmit power, the control means monitors the SINR indicator derived at the intermediate apparatus to determine if the commanded change in transmit power of the source apparatus has been effected. Then:

5a. if it is determined that the change was not effected by the UE, the control means places a prohibition on further increases in the transmit power of the RN; or

5b. If it is determined that the change was effected by the UE, the control means lifts any existing prohibition on an increase in the transmit power of the RN.

FIG. 5 shows parts of a base station embodying the first and second aspects of the present invention and comprises:

-   -   indicator derivation means (1), operable to derive one or more         indicators of the quality of a communication signal received at         the base station; indicator deviation detection means (2)         operable to detect a change in the, or one of the, indicators         derived by the base station; indicator receiving means (3),         operable to receive an indicator derived by the intermediate         apparatus;     -   imbalance detection means (4) operable to detect an imbalance         between an indicator derived by the indicator derivation means         and an indicator received by the indicator receiving means;     -   determining means (5), operable to determine a change in the         transmit power of the intermediate apparatus and/or a change in         the transmit power of the source apparatus, as the case may be,         following detection of an indicator deviation by the indicator         deviation detection means (3) and/or following detection of an         imbalance by the imbalance detection means (4); and     -   control means (6), operable to receive a request from the         determining means and, subject to various checks being performed         by the control means, to issue a command to the intermediate         apparatus and/or the source apparatus, as the case may be,         commanding a change in the transmit power of the intermediate         apparatus and/or the source apparatus respectively.

Following the detection of an imbalance and the issuance of a command to the source apparatus for an increase in the transmit power of the source apparatus, the control means (6) is operable to prohibit an increase in the transmit power of the intermediate apparatus (output) if it is detected by the control means that the command for an increase in the transmit power of the source apparatus was not met. Then, following the detection of a change in the indicator derived by the base station, and prior to issuing a command to the intermediate apparatus for an increase in the transmit power of the intermediate apparatus, the control means is operable to check (input) if any prohibition has been placed on increases in the transmit power of the intermediate apparatus.

Theoretical Analysis

Although embodiments of the preset invention seek to balance the quality indicators derived by the destination apparatus and intermediate apparatus without performing an explicitly calculation of the transmit power required to achieve that balance, the following theoretical analysis, which derives possible solutions for explicitly calculating the optimal transmit power of the transmitting elements comprised in a multi-hop network for various deployment scenarios, is useful for understanding the present invention. Whilst the equations are developed solely for the case of the connections that form the downlink in a multi-hop network, it is straightforward to adapt the equations derived for the case of the uplink. Such an adaptation is achieved by adopting the same methodology used to develop the expressions for the received SINR at the receiving nodes, where the transmitting nodes are now the UE and the RN and the receiving nodes are now the NB and RN. Once expressions for the SINR received at the RN and NB are arrived at, the same methodology can be employed for each deployment scenario in order to determine the optimal transmit power setting of the UE and RN. For each deployment scenario, theoretical solutions are obtained assuming a single-cell model and a two-cell model. In the case of a two cell model, it is assumed that the deployment in both cells is identical and that the transmit powers on the bas station (BS) and the intermediate apparatus (I) are the same. It is also assumed that where appropriate P_(tx) _(—) _(tot,RN)=G_(p)P_(tx,RN) and P_(tx) _(—) _(tot,NB)=G_(p)P_(tx,NB) and that for the case of TDD both RN's transmit at the same time. This in effect generates the worse case scenario for two cells.

Theoretical solutions may be evolved from a consideration of the signal-to-interference plus noise ratio (SINR) experienced by the receiving nodes in a multi-hop system (i.e. the or each intermediate apparatus (I) and the destination apparatus (D)). The SINR at a particular node is a measure of the quality of a communication signal received by that node and is a ratio of the received strength of the desired signal to the received signal strength of the undesired signals (noise and interference).

As previously discussed, the considerations required for noise and interference depend on the duplexing method used to separate signal received at an intermediate apparatus from those transmitted from an intermediate apparatus, the characteristics of the intermediate apparatus and also the level of inter-cell interference which is taken into account (i.e. interference from neighbouring cells).

The following equation represents the SINR of a communication signal sent from an intermediate apparatus to a destination apparatus for all scenarios, where different terms may be ignored depending upon the type of intermediate apparatus (e.g. non-regenerative or regenerative) and the duplexing method: ${SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}$

For the case of FDD instead of TDD then the third term in the bracket is removed and for the case of regenerative instead of non-regenerative the second term in the bracket is removed.

In the case of a two-cell model as illustrated in FIG. 1B, this becomes: ${SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{{RN}\quad 1}}}{L_{{{RN}\quad 1} - {UE}}\begin{pmatrix} \begin{matrix} {N + \frac{P_{{tx},{{RN}\quad 1}}}{L_{{{RN}\quad 1} - {UE}}{SINR}_{{{NB}\quad 1} - {{RN}\quad 1}}} +} \\ {\frac{\quad P_{{tx\_ tot},\quad{{NB}\quad 1}}}{\quad L_{\quad{{{NB}\quad 1}\quad - \quad{UE}}}} + \frac{\quad P_{{tx\_ tot},\quad{{NB}\quad 2}}}{\quad L_{\quad{{{NB}\quad 2}\quad - \quad{UE}}}} +} \end{matrix} \\ \frac{P_{{tx\_ tot},{{RN}\quad 2}}}{L_{{{RN}\quad 2} - {UE}}} \end{pmatrix}}$

The first three terms in the bracket in (2) are the same as those in (1). The additional last two terms originate from the interference experienced from the neighbouring co-channel NB and RN respectively. Obviously if the neighbouring cell employs a different frequency or uses a different timeslot for relay transmission then the terms needed model this interference will vary. It should be appreciated that these equations can be extended to a three-cell model or more for a higher level of accuracy.

Considering now the various possible deployment scenarios in turn, for the case of DL transmissions transmitted between a base-station or node-B (NB), via an intermediate relay node (RN) to a destination user equipment (UE).

1A. Regenerative Relay with FDD—Single-Cell Model as Illustrated in FIG. 1A

In this case, the SINR at a destination UE which is connected to an intermediate RN is given by: $\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}N}} & (1) \end{matrix}$

Where G_(p) is the processing gain, P_(tx,RN) is the transmit power on the channel of interest at the RN, L_(RN-UE) is the propagation loss on the NB to RN link and N is the noise. Note this assumes that no intra-cell interference exists.

The SINR at an intermediate RN which is operable to receive signals from the NB is given by: $\begin{matrix} {{SINR}_{{NB} - {RN}} = \frac{G_{p}P_{{tx},{NB}}}{L_{{NB} - {RN}}N}} & (2) \end{matrix}$

Where P_(tx,NB) is the transmit power on the channel of interest at the NB and L_(NB-RN) is the propagation loss on the RN to UE link. Again, it is assumed that no intra-cell interference exists.

The overall throughput across the multi-hop link will be limited by the lower of the two SINR values as this will limit the rate at which data can be transmitted to that entity. Any increase in transmit power that causes an SINR imbalance will not improve the performance of the multi-hop system; it will simply result in wasted energy and an increase in interference to any co-channel users.

Thus, assuming that the receiver at the intermediate RN and the receiver at the destination UE perform the same, then it follows that the transmit power at the NB and RN should be set such that the SINR at the RN and UE is the same. Using this criterion for setting the ratio of the transmit powers, it follows that the ratio is given by: $\begin{matrix} \begin{matrix} {\frac{P_{{tx},{NB}}}{P_{{tx},{RN}}} = \frac{L_{{NB} - {RN}}}{L_{{RN} - {UE}}}} \\ {= \frac{b_{1}s_{1}^{n_{1}}}{b_{2}s_{2}^{n_{2}}}} \end{matrix} & (3) \end{matrix}$

Where b₁ and n₁ are the pathloss parameters for the NB to RN link which is s₁ in length and b₂, n₂ and s₂ are associated with the RN to UE link. Thus using equation (3) it is possible to find either transmit power given the other.

1 B. Regenerative Relay with FDD—Two Cell Model as Shown in FIG. 1B

In this case, transmit power equations may be derived taking into account interference caused by transmissions arising in the other cell.

In this case the SINR at a destination UE that is operable to receive signals from an intermediate RN is now: $\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} & (4) \end{matrix}$

The optimal NB transmit power can be found by setting (4) and (2) to be equal. Therefore: $\begin{matrix} \begin{matrix} {P_{{tx},{NB}} = \frac{L_{{NB} - {RN}}N\quad P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} \\ {= \frac{L_{{NB} - {RN}}P_{{tx},{RN}}}{\left( {L_{{RN} - {UE}} + \frac{G_{p}P_{{tx},{RN}}}{N}} \right)}} \end{matrix} & (5) \end{matrix}$ (5) can be rearranged to find the intermediate RN transmit power given the source NB transmit power: $\begin{matrix} {P_{{tx},{RN}} = \frac{L_{{RN} - {UE}}}{\left( {\frac{L_{{NB} - {RN}}}{P_{{tx},{NB}}} - \frac{G_{p}}{N}} \right)}} & (6) \end{matrix}$ 2A. Regenerative Relay with TDD: Single Cell Model—FIG. 1A

It is assumed that the two links (source to intermediate, intermediate to destination) operate on the same frequency with TDD being used to separate the receive and transmit operation of the RN (i.e. it is no longer full duplex). If it is assumed that the timeslot in which the RN transmits is not used by the NB then the equations described above for the case of a regenerative relay with an FDD duplexing scheme can be used. However, if the source NB uses the same timeslot as the intermediate RN to communicate with apparatuses or nodes other than the NB, interference will result to the transmission made by the RN. In this case the SINR at a destination UE that is operable to receive communication signals from an intermediate RN is given by: $\begin{matrix} \begin{matrix} {{SINR}_{{RN}\quad - \quad{UE}} = \frac{G_{p}\quad P_{{tx},\quad{RN}}}{L_{{RN}\quad - \quad{UE}}\left( {N + I} \right)}} \\ {= \frac{G_{p}\quad P_{{tx},\quad{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{{tx\_ to}t},{NB}}}{L_{{NB} - {UE}}}} \right)}} \end{matrix} & (7) \end{matrix}$

Where P_(tx) _(—) _(tot,NB) is the total transmission power from the NB and L_(NB-UE) is the propagation loss on the NB to UE link. In this case the transmit power at the RN that ensures equal SINR is given by: $\begin{matrix} {P_{{tx},{RN}} = {{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{P_{{{tx\_ to}t},{NB}}}{N\quad L_{{NB} - {UE}}}} \right)}} & (8) \end{matrix}$

Comparing equation (3) and equation (8) it is apparent that a simple ratio no longer yields the ideal balance. Assuming that P_(tx) _(—) _(tot,NB)=G_(p)P_(tx,NB) it is possible to write equation (8) as: $\begin{matrix} \begin{matrix} {P_{{tx},{RN}} = {{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {UE}}}} \right)}} \\ {= {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {P_{{tx},{NB}} + \frac{G_{p}P_{{tx},{NB}}^{2}}{{NL}_{{NB} - {UE}}}} \right)}} \end{matrix} & (9) \end{matrix}$

From (9) it is possible to determine the ideal RN transmit power given the NB transmit power. It is worth noting that if the set-up of the system is arranged such that the second term in the second bracket is negligible (i.e. P_(tx) _(—) _(tot,NB)/NL_(NB-UE)<<1) then the criterion described above for the case of a regenerative relay with an FDD duplex scheme can be used.

It follows that the ideal NB transmit power given a certain RN transmit power can be found from the roots of (9). Expressing (9) in the following simplified form: $\begin{matrix} {{{{\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}P_{{tx},{NB}}} + {\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}\frac{G_{p}}{{NL}_{{NB} - {UE}}}P_{{tx},{NB}}^{2}} - P_{{tx},{RN}}} = 0}{{{ax}^{2} + {bx} + c} = 0}} & (10) \end{matrix}$

Where ${x = P_{{tx},{NB}}},{a = \frac{G_{p}L_{{RN} - {UE}}}{{NL}_{{NB} - {RN}}L_{{NB} - {UE}}}},{b = {{\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}{and}\quad c} = {- P_{{tc},{RN}}}}}$ it follows that the roots of (10) are given by: $\begin{matrix} {x = \frac{{- b} \pm \sqrt{b^{2} - {4{ac}}}}{2a}} & (11) \end{matrix}$

As the transmit power is a positive number, only one root is defined, it therefore follows that the optimal transmit power at the NB that ensures equal SINR at the RN and UE is given by: $\begin{matrix} {x = {P_{{tx},{NB}} = \frac{{- b} + \sqrt{b^{2} + {4{aP}_{{tx},{RN}}}}}{2a}}} & (12) \end{matrix}$

Finally, it is possible to use the definitions above to rewrite (9), which gives the optimal RN transmit power, in a similar simplified form: P_(tx,RN) =bP _(tx,NB) +aP _(tx,NB) ²  (13) 2A. Regenerative Relay with TDD: Two-Cell Model as Shown in FIG. 1B

In addition to assuming that the deployment in both is identical and that the transmit powers on the NB and RN are the same, it is also assumed that where appropriate P_(tx) _(tot,RN) =G_(p)P_(tx,RN) and P_(tx) _(—) _(tot,NB)=G_(p)P_(tx,NB) and that for the case of TDD both RN's transmit at the same time. This in effect generates the worse case scenario for two cells.

In this case the SINR at the destination UE that is operable to receive signals from an intermediate RN is now: $\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{2G_{p}P_{{tx},{NB}}}{L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} & (14) \end{matrix}$

The optimal NB transmit power can be found by setting (14) and (2) to be equal: $\begin{matrix} {{\frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {RN}}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{2G_{p}P_{{tx},{NB}}}{L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}}{P_{{tx},{RN}} = {{{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{2P_{{tx\_ tot},{NB}}}{{NL}_{{NB} - {UE}}} + \frac{P_{{tx\_ tot},{RN}}}{{NL}_{{RN} - {UE}}}} \right)\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( \frac{2G_{p}}{{NL}_{{NB} - {UE}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)P_{{tx},{NB}}} - P_{{tx},{RN}}}}} & (15) \end{matrix}$

The optimal NB transmit power is found from the positive root of: $\begin{matrix} {{{\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( \frac{2G_{p}}{{NL}_{{NB} - {UE}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)P_{{tx},{NB}}} - P_{{tx},{RN}}} = 0} & (16) \end{matrix}$

Which is given by: $\begin{matrix} {x = {P_{{tx},{NB}} = \frac{{- b} + \sqrt{b^{2} - {4\quad{ac}}}}{2a}}} & (17) \end{matrix}$

Where in this case $\begin{matrix} {{{a = \frac{2G_{p}L_{{RN} - {UE}}}{{NL}_{{NB} - {RN}}L_{{NB} - {UE}}}},{b = {\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)}}}{and}{{c = {- P_{{tx},{RN}}}},}} & \quad \end{matrix}$ and both b and c are a function of the RN transmit power.

Given the NB transmit power it is possible to rearrange (15) to find the RN transmit. It follows that the optimal RN transmit power is given by: $\begin{matrix} {P_{{tx},{RN}} = \frac{{\left( {\frac{2G_{p}}{{NL}_{{NB} - {UE}}}\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)P_{{tx},{NB}}}}{1 - {\left( {\frac{G_{p}}{{NL}_{{RN} - {UE}}}\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}} \right)P_{{tx},{NB}}}}} & (18) \end{matrix}$ 3A. Non-Regenerative Relay Node (RN) with FDD—Single Cell Model as Shown in FIG. 1A

The difference between this case and that of a regenerative relay node being used in conjunction with a FDD duplexing scheme is that the SINR at the UE is a function of the SINR at the RN, where the SINR at the destination UE which is connected to the RN is given by: $\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{ix},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{ix},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}}} \right)}} & (19) \end{matrix}$

The result is that the ideal balance is no longer derived from setting the SINR at the UE equal to that at the RN. According to (19), the SINR at the RN needs to be set so that it does not prevent this target SINR at the UE from being obtained. However, the NB power must be controlled to limit the SINR at the RN rising beyond that practically required else excess interference and wasted transmit power will result.

FIG. 6 illustrates how the setting of NB and RN transmit power affects the SINR at the UE connected to the RN for a two different deployment scenarios.

Thus, it can be seen that the optimal solution is to select the transmit power of the NB and RN such that the system effectively operates on the diagonal fold in the surface shown in FIG. 6. It is possible to realise such a solution by taking the first derivative of (19) and finding the point at which increasing either the NB or RN transmit power results in minimal increase to SINR at UE.

In order to determine the first derivative of (19), it is rewritten as: $\begin{matrix} \begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{ix},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{ix},{RN}}}{L_{{RN} - {UE}}\frac{G_{p}P_{{ix},{NB}}}{\quad{NL}_{{NB} - {RN}}}}} \right)}} \\ {= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{ix},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{ix},{NB}}} \right)}} \end{matrix} & (20) \end{matrix}$

Defining ${y = {SINR}_{{RN} - {UE}}},{k_{1} = \frac{{NL}_{{RN} - {UE}}}{G_{p}}}$ and $k_{2} = \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}}$ it is possible to simplify (20) to be: $\begin{matrix} {y = {\frac{1}{\frac{k_{1}}{P_{{ix},{RN}}} + \frac{k_{2}}{P_{{ix},{NB}}}} = \frac{P_{{ix},{NB}}}{\frac{k_{1}P_{{ix},{NB}}}{P_{{ix},{RN}}} + k_{2}}}} & (21) \end{matrix}$

In order to find the rate of change of SINR with P_(tx,NB) the quotient rule for differentiation is used: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{ix},{NB}} \right)} = {\frac{k_{2}}{\left( {{\frac{k_{1}}{P_{{ix},{RN}}}P_{{ix},{NB}}} + k_{2}} \right)^{2}} = \nabla_{NB}}} & (22) \end{matrix}$

By solving (22) for P_(tx,NB) given the required gradient and P_(tx,RN) it is possible to find the optimal NB transmit power: $\begin{matrix} {P_{{ix},{NB}} = \frac{P_{{ic},{NB}}\left( {\sqrt{\frac{k_{2}}{\nabla_{NB}}} - k_{2}} \right)}{k_{1}}} & (23) \end{matrix}$

In order to find the optimal RN transmit power given that of the NB, the differentiation of (21) is now performed with respect to P_(tx,RN). In this case the first order derivative is given by: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{ix},{NB}} \right)} = {\frac{k_{1}}{\left( {{\frac{k_{2}}{P_{{ix},{RN}}}P_{{ix},{RN}}} + k_{1}} \right)^{2}} = \nabla_{RN}}} & (24) \end{matrix}$

And the optimal RN transmit power given that of the NB is: $\begin{matrix} {P_{{ix},{RN}} = \frac{P_{{ic},{NB}}\left( {\sqrt{\frac{k_{1}}{\nabla_{RN}}} - k_{1}} \right)}{k_{2}}} & (25) \end{matrix}$ 3B. Non-Regenerative Relay Node (RN) with FDD—Two Cell Model as Shown in FIG. 1B

In a two cell model the SINR for the worse case of a destination UE at the cell edge is given by: $\begin{matrix} \begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{ix},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{ix},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}} + \frac{G_{p}P_{{ix},{RN}}}{L_{{RN} - {UE}}}} \right)}} \\ {= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{ix},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{ix},{NB}}} \right) + 1}} \end{matrix} & (26) \end{matrix}$

Assuming that the transmit power of the two RN's is equal, the deployment is identical across the two cells and that P_(tx) _(—) _(tot,RN)=G_(p)P_(tx,RN), then the simplified form of (26) is given by: $\begin{matrix} \begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{1}{\frac{k_{1}}{P_{{ix},{RN}}} + \frac{k_{2}}{P_{{ix},{NB}}} + 1}} \\ {= \frac{P_{{ix},{NB}}}{{\left( {\frac{k_{1}}{P_{{ix},{RN}}} + 1} \right)P_{{ix},{NB}}} + k_{2}}} \end{matrix} & (27) \end{matrix}$

The first derivative is now: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{NB}} \right)} = \frac{k_{2}}{\left( {{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2}} \right)^{2}}} & (28) \end{matrix}$

Thus the optimal NB transmit power can be found by: $\begin{matrix} {P_{{tx},{NB}} = \frac{{P_{{tx},{RN}}\sqrt{\frac{k_{2}}{\nabla}}} - k_{2}}{k_{1} + P_{{tx},{RN}}}} & (29) \end{matrix}$

The optimal RN transmit power is found by taking the derivative of (27) with respect to P_(tx,RN): $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{RN}} \right)} = \frac{k_{1}}{\left( {{\left( {\frac{k_{2}}{P_{{tx},{NB}}} + 1} \right)P_{{tx},{RN}}} + k_{1}} \right)^{2}}} & (30) \end{matrix}$

Thus the optimal RN transmit power can be found by: $\begin{matrix} {P_{{tx},{RN}} = \frac{{P_{{tx},{NB}}\sqrt{\frac{k_{1}}{\nabla}}} - k_{1}}{k_{2} + P_{{tx},{NB}}}} & (31) \end{matrix}$ 4A—Non-Regenerative Relay with TDD—Single Cell Model as Shown in FIG. 1A

This case is similar to that described above for a non-regenerative except for the fact that now interference from the NB must be taken into account due to the fact that it transmits on the same frequency and at the same time as the RN. In this case the SINR at the UE which is receiving communication signals transmitted by the RN is given by: $\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}} & (32) \end{matrix}$

If the P_(tx,NB)/P_(tx,RN) is too large the SINR at the UE is limited due to insufficient RN transmit power and it is likely the area in which the link performance of a connection to a RN outperforms that for a connection to the NB is reduced. Conversely, if it is too small then the SINR at the UE is limited by the low SINR at the RN.

In this case, the balance is even finer than of that described in the case of a non-regenerative relay node employed in conjunction with an FDD duplexing scheme, as illustrated by FIG. 7. The optimal operating point is given by finding the point at which the first derivative of (32) is equal to zero. In order to find this optimal point, (32) is first rearranged in the following form: $\begin{matrix} {\begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( \frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {RN}}} \right)} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}} \\ {= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{tx},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{tx},{NB}}} \right) + \left( \frac{L_{{RN} - {UE}}P_{{tx},{NB}}}{L_{{NB} - {UE}}P_{{tx},{RN}}} \right)}} \end{matrix}\quad} & (33) \end{matrix}$

Defining ${{y = {SINR}_{{RN} - {UE}}},{k_{1} = {\frac{{NL}_{{RN} - {UE}}}{G_{p}}\quad{and}}}}\quad$ $k_{2} = \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}}$

Using the definitions from described in 3A above and $k_{3} = \left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {UE}}} \right)$ it is possible to simplify (33) to: $\begin{matrix} {\begin{matrix} {y = \frac{1}{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right) + \left( \frac{k_{2}}{P_{{tx},{NB}}} \right) + \left( \frac{k_{3}P_{{tx},{NB}}}{P_{{tx},{RN}}} \right)}} \\ {= \frac{P_{{tx},{NB}}}{{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right)P_{{tx},{NB}}} + k_{2} + {\left( \frac{k_{3}}{P_{{tx},{RN}}} \right)P_{{tx},{NB}}^{2}}}} \end{matrix}\quad} & (34) \end{matrix}$

The next step is to find the single maxima of the parabolic function in (34) by solving: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}x} = 0} & (35) \end{matrix}$

Using the quotient rule to find the first derivative of (34): $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{NB}} \right)} = \frac{\begin{matrix} {{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}} - P_{{tx},{NB}}} \\ \left( {\frac{k_{1}}{P_{{tx},{RN}}} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right) \end{matrix}}{\left( {{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \right)^{2}}} & (36) \end{matrix}$

The maxima of y is found by setting (36) equal to zero and solving for P_(tx,NB). It follows that the maximum SINR at the UE is obtained by setting: $\begin{matrix} {{{{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {P_{{tx},{NB}}^{2}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \right)}}{P_{{tx},{NB}} = \sqrt{\frac{P_{{tx},{RN}}k_{2}}{2k_{3}}}}} & (37) \end{matrix}$

Therefore, given the transmit power of the RN it is possible to use (37) to find the corresponding NB transmit power that ensures maximum SINR at the UE that is connected to the RN.

For the case of finding the optimal RN transmit power given the NB transmit power a similar approach to that described in above in the case of a non-regenerative relay node employed in conjunction with an FDD duplexing scheme, can be used as the SINR at the UE is not a parabolic function of RN transmit power. In order to find the optimal RN transmit power, (34) is rearranged to the following: $\begin{matrix} \begin{matrix} {y = \frac{1}{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right) + \left( \frac{k_{2}}{P_{{tx},{NB}}} \right) + \left( \frac{k_{3}P_{{tx},{NB}}}{P_{{tx},{RN}}} \right)}} \\ {= \frac{P_{{ix},{RN}}}{\left( \frac{P_{{tx},{RN}}k_{2}}{P_{{tx},{NB}}} \right) + {k_{3}P_{{tx},{NB}}} + k_{1}}} \end{matrix} & (38) \end{matrix}$

The first derivative is now: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{RN}} \right)} = {\frac{{k_{3}P_{{tx},{NB}}} + k_{1}}{\left( {\left( \frac{P_{{tx},{RN}}k_{2}}{P_{{tx},{NB}}} \right) + {k_{3}P_{{tx},{NB}}} + k_{1}} \right)^{2}} = \nabla}} & (39) \end{matrix}$

Solving (39) for P_(tx,RN) gives the optimal RN transmit power given the NB transmit power: $\begin{matrix} {P_{{tx},{RN}} = \frac{P_{{tx},{NB}}\left( {\sqrt{\frac{{k_{3}P_{{tx},{NB}}} + k_{1}}{\nabla}} - \left( {{k_{3}P_{{tx},{NB}}} + k_{1}} \right)} \right)}{k_{2}}} & (40) \end{matrix}$

By observing the surface in FIG. 7 and from the form of (34) and the result in (40) it is apparent that if the NB transmit power is small then the rate of change of SINR with RN transmit power will decrease with increasing RN transmit power. However, for the case of large NB transmit power, the SINR at the UE approximates to a linear function of RN transmit power. The result is that in this case the solution to the problem, as summarised in (40) will be infinite.

4B—Non-Regenerative Relay with TDD—Two Cell Model as Shown in FIG. 1B

The worse case, from the perspective of a UE at the cell edge, is when the neighbouring cell employs a TDD scheme with the same timeslot used for RN transmission. If it is assumed that the cells are equal in size with the same deployment and transmit power settings and that P_(tx) _(—) _(tot,RN/NB)=G_(p)P_(tx,RN/NB) then: $\begin{matrix} \begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\begin{pmatrix} {N + \frac{\quad P_{\quad{{tx},\quad{RN}}}}{\quad{L_{\quad{{RN}\quad - \quad{UE}}}\quad{SINR}_{\quad{{NB}\quad - \quad{R\quad 1}}}}} +} \\ {\frac{2\quad G_{\quad p}\quad P_{\quad{{tx},\quad{NB}}}}{\quad L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \end{pmatrix}}} \\ {= \frac{1}{\left( \frac{\quad{NL}_{\quad{{RN}\quad - \quad{UE}}}}{\quad{G_{\quad p}\quad P_{\quad{{tx},\quad{RN}}}}} \right) + \left( \frac{\quad{NL}_{\quad{{NB}\quad - \quad{RN}}}}{\quad{G_{\quad p}^{\quad 2}\quad P_{\quad{{tx},\quad{NB}}}}} \right) +}} \\ {\left( \frac{2\quad L_{\quad{{RN}\quad - \quad{UE}}}\quad P_{{tx},\quad{NB}}}{\quad{L_{\quad{{NB}\quad - \quad{UE}}}\quad P_{\quad{{tx},\quad{RN}}}}} \right) + 1} \end{matrix} & (41) \end{matrix}$

In this case the simplified form of (4) is: $\begin{matrix} \begin{matrix} {{SINR}_{{RN} - {UE}} = \frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + 1}} \\ {= \frac{P_{{tx},{NB}}}{{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}}} \end{matrix} & (42) \end{matrix}$

And the first derivative is: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{NB}} \right)} = \frac{\begin{matrix} {{\left( {\frac{\quad k_{1}}{\quad P_{{tx},{RN}}} + 1} \right)P_{\quad{{tx},\quad{NB}}}} + k_{2} + {\frac{2\quad k_{3}}{\quad P_{{tx},{RN}}}P_{{tx},{NB}}^{2}} -} \\ {P_{{tx},{NB}}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1 + {\frac{4k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right)} \end{matrix}}{\begin{pmatrix} {{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} +} \\ {k_{2} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \end{pmatrix}^{2}}} & (43) \end{matrix}$

Finally, the maxima is given by setting (43) equal to zero and solving for P_(tx,NB): $\begin{matrix} {{{{\left( {\frac{\quad k_{1}}{\quad P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {P_{{tx},{NB}}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1 + {\frac{4k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right)}}{{k_{2} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {\frac{4k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}}{P_{{tx},{NB}} = \sqrt{\frac{P_{{tx},{RN}}k_{2}}{2k_{3}}}}} & (44) \end{matrix}$

In order to find the optimal RN transmit power given the NB transmit power (42) is rearranged to: $\begin{matrix} \begin{matrix} {y = \frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + 1}} \\ {= \frac{P_{{ix},{RN}}}{k_{1} + \frac{k_{2}P_{{tx},{RN}}}{P_{{tx},{NB}}} + {2k_{3}P_{{tx},{NB}}} + P_{{tx},{RN}}}} \end{matrix} & (45) \end{matrix}$

The first derivative is now: $\begin{matrix} {\frac{\mathbb{d}y}{\mathbb{d}\left( P_{{tx},{RN}} \right)} = {\frac{k_{1} + {2k_{3}P_{{tx},{NB}}}}{\left( {k_{1} + {2k_{3}P_{{tx},{NB}}} + {P_{{tx},{RN}}\left( {1 + \frac{k_{2}}{P_{{tx},{NB}}}} \right)}} \right)^{2}} = \nabla}} & (46) \end{matrix}$

Solving (46) for P_(tx,RN) gives the optimal RN transmit power given the NB transmit power: $\begin{matrix} {P_{{tx},{RN}} = \frac{{P_{{tx},{NB}}\sqrt{\frac{k_{1} + {2k_{3}P_{{tx},{nB}}}}{\nabla}}} - \left( {k_{1} + {2k_{3}P_{{tx},{NB}}}} \right)}{\left( {P_{{tx},{NB}} + k_{2}} \right)}} & (47) \end{matrix}$

Again, in the case of large NB transmit power, the SINR at the UZ approximates to a linear function of RN transmit power. The result is that the solution to (47) will be infinite.

The optimal transmit power balance will now be determined based on the solutions developed above for the different relay and duplexing schemes and for two separate deployment scenarios. These deployment scenarios are summarised in Table III and the propagation parameters of the pathloss equation in (48) are in Table IV. L=b+10n log d  (48)

Where L is the pathloss in dB, b is in dB and is given in Table along with n, and d is the transmitter-receiver separation in metres. TABLE III Deployment scenarios Scenario Parameter 1 2 Cell Radius 1867 m Relay Position 933 m 1400 m

The transmitter receiver separation is the same as the cell radius (i.e. the UE is located at the cell radius). The RN position quoted is relative to the centre of the cell which is where the NB is located. The RN positions are therefore the distance from the NB to the RN. The RN-UE is then the difference of the cell radius and the NB-RN separation. TABLE V Propagation parameters. Link Parameter NB-UE NB-RN RN-UE b (dB) 15.3 15.5 28 n 3.76 3.68 4 Regenerative Relay

Substituting the values given in Table III and Table IV into equations (3) and (5) for FDD and (12) and (17) for TDD it is possible to find the optimal NB transmit power given the RN transmit power. FIG. 8A shows the optimal NB transmit power as a function of RN transmit power for both FDD and TDD for the two deployment scenarios.

Non-Regenerative Relay with FDD

Substituting the parameters into (23) and (24) it is possible to find the optimal NB transmit power for the two deployment scenarios, as shown in FIG. 8B.

Non-Regenerative Relay with TDD

Substituting the parameters into (37) and (44) it is possible to find the optimal NB transmit power for the two deployment scenarios, as shown in FIG. 8C.

System Level Simulation Results

System simulation of a multi-hop HSDPA network employing non-regenerative relays with TDD duplexing with relays transmitting in every third transmission time interval have been conducted in order to validate the predicted throughput gain based on results of FIG. 8C, with the average packet call throughput gain being determined as the transmit powers of the RN and NB are varied around the optimal point.

Results of a system level simulation for the two deployment scenarios detailed above in Table III will now be presented. The simulation parameters are listed below in Table V and Table VI. TABLE V Deployment parameters Parameter Value Base Station Inter-cell Separation 2.8 km Sectors/cell 3 Antenna Height 15 m Antenna Gain 17 dBi Relay Station RN antenna 120° position ½ and ¾ cell radius Num/cell 9 Antenna Height 5 m Antenna Gain 17 dBi User Number per sector 50 Equipment Initial Distribution Random Velocity 3 km/h Direction Semi-directed Update 20 m Traffic Models WWW

TABLE VI Simulation parameters Parameters Value Base Station/ HS-DSCH power Variable Relay Node CPICH power 20% of total HARQ scheme Chase HS-DSCH/frame 15 Relay buffer size 1.78 Mbits Ack/NAck Detection Error free NB Scheduler Round Robin Relay type Amplify & Forward User Thermal Noise Density 10-174 dBm/Hz Equipment Noise Figure 5 dBm Detector MMSE

For both deployment scenarios the gain in the average packet call throughput experienced by the users on that observed for the case of a single hop system with NB transmission power of 30 dBm is plotted as a function of NB transmit power for four different RN transmit powers. FIG. 9A shows the gain for deployment scenario 1 and FIG. 9B shows the gain for scenario 2.

Note that the channel gain for the NB to UE link was 3 dB higher than for the NB to RN and RN to UE link. This means that the interference experienced by a UE connected to a RN from another NB is double that used in the link analysis discussed above with reference to FIGS. 8A, 8B and 8C. The channel gain is due to the fact that a number of replicas of the transmitted signal are received, when the power on all these is added it is found that for the case of the NB to UE channel the total power is double that on the NB to RN or RN to UE channel. This accounts for the 3 dB gain, as 3 dB equates to double. As a result of the channel gain being higher for the NB to UE channel, this means that the received signal power will be 3 dB (or double) higher than that used in the analysis up to that point where no channel gain through multi-path was considered.

Comparison of Link Based Prediction and System Simulation

FIG. 10 shows the optimal NB transmit power as a function of RN transmit power for a non-regenerative relay for TDD for each deployment scenario where it is assumed the NB to UE link has a 3 dB gain compared with the other links. In this case, the predicted transmit power at the NB for the RN transmit power used in the simulation are listed in Table VII along with the throughput gain that would be experienced if these settings were used and the maximum achievable. TABLE IIII Predicted optimal NB transmit power and resulting simulated throughput gain that would have been achieved from this setting compared with the maximum gain observed. NB Transmit Power (dBm) & User Packet Throughput Gain Scenario 1 Scenario 2 RN Transmit Throughput Throughput Power (dBm) Predicted Gain Max Gain Predicted Gain Max Gain 16 −0.5 33% 40% 8.8 60% 67% 19 1 38% 43% 10.3 65% 74% 22 2.5 41% 46% 11.8 68% 74% 25 4 49% 51% 13.3 72% 75%

Table VII, FIG. 8A and FIG. 9B suggest that if power balancing is performed according to a preferred embodiment of the present invention using a technique based on the equations developed above then the selected power balance will in general be in the region of the optimal point. In particular, for the transmit powers used the gain was shown to always be within 10% of the achievable maximum, with the difference being due to shortcomings of using of a two-cell model to model a multi-cell system.

The necessity of transmit power balancing is apparent in the results presented in both FIG. 9A and FIG. 9B where it is shown that if the NB transmit is increased beyond the optimal point then a significant degradation in gain will be experienced despite the emission of more signal energy. It also shows that if the NB transmit power is selected carefully then the sensitivity of the gain to RN transmit power is reduced. 

1. A communication system comprising a source apparatus, an intermediate apparatus and a base station, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the base station comprising an indicator derivator operable to derive one or more indicators of the quality of a communication signal received at the base station, and wherein the communication system further comprises: i) a controller provided in the base station; ii) an indicator deviator detector operable to detect a change in one said indicator derived by the base station from a desired value; iii) a determination unit operable, following the detection of such a change, to determine a required change in the transmit power of the intermediate apparatus that will tend to bring said indicator to said desired value, wherein said determination unit further comprises a request transmitter operable to transmit a request for a change in the transmit power of the intermediate apparatus to the controller.
 2. A communication system as claimed in claim 1, wherein one said indicator derived by said destination apparatus comprises a measure of the strength of a communication signal received at the base station.
 3. A communication system as claimed in claim 1, wherein one said indicator derived by said destination apparatus comprises a measure of the signal-to-interference plus noise ratio (SINR) of a communication signal received at the base station.
 4. A communication system as claimed in claim 1, wherein one said indicator derived by said base station comprises a measure of the variation of the quality of a communication signal received at the destination apparatus from a target received signal quality.
 5. A communication system as claimed in claim 1, wherein if the required change in transmit power of the intermediate apparatus comprises a decrease in transmit power, said controller is operable, following receipt of a request for a change in the transmit power of the intermediate apparatus, to issue a command to said intermediate apparatus, commanding a change in transmit power of the intermediate apparatus.
 6. A communication system as claimed in claim 1, wherein the controller further comprises an input signal receiver, operable to receive an input signal which allows the controller to determine if an increase in transmit power of the intermediate apparatus is prohibited.
 7. A communication system as claimed in claim 6, wherein if the required change in transmit power of the intermediate apparatus comprises an increase in transmit power, and following a determination by the controller that an increase in transmit power of the intermediate apparatus is prohibited, the controller is operable to ignore said request.
 8. A communication system as claimed in claim 6, wherein if the required change in transmit power of the intermediate apparatus comprises an increase in transmit power, and following a determination by the controller that an increase in transmit power of the intermediate apparatus is not prohibited, the controller is operable issue a command to said intermediate apparatus, commanding a change in transmit power of the intermediate apparatus.
 9. A communication system as claimed in claim 5, wherein said intermediate apparatus is operable to receive such a command from said controller of the base station.
 10. A communication system as claimed in claim 9, wherein said intermediate apparatus is operable to determine, based on a maximum transmit power of the intermediate apparatus, if said intermediate apparatus can carry out the change in transmit power according to said command.
 11. A communication system as claimed in claim 10, wherein if said intermediate apparatus determines that it cannot carry out the change in transmit power according to said command, said intermediate apparatus is operable to determine a revised change in transmit power of the intermediate apparatus which can be carried out by said intermediate apparatus.
 12. A communication system as claimed claim 9, wherein said intermediate apparatus is operable to cause the transmit power of the intermediate apparatus to change in accordance with said request, or said revised request, as the case may be.
 13. A communication system as claimed in claim 1, wherein said intermediate apparatus comprises an indicator derivator operable to derive an indicator of the quality of a communication signal received at the intermediate apparatus and wherein said intermediate apparatus is further operable to transmit said indicator to an indicator receiver of said base station, wherein said base station further comprises an imbalance detector operable to detect an imbalance between an indicator derived by the base station and an indicator derived by the intermediate apparatus, and wherein said determination unit is further operable, following detection of such an imbalance by said imbalance detector, to determine a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance, wherein said controller is further operable, following determination of said change, to issue a command to said source apparatus commanding a change in the transmit power of the source apparatus in accordance said required change.
 14. A communication system as claimed in claim 13, wherein said source apparatus is operable, following receipt of a command, and wherein said command is for an increase in transmit power, to determine if it can carry out the command based on the maximum transmit power of the source apparatus.
 15. A communication system as claimed in claim 14, wherein if said source apparatus determines that it cannot carry out the said request, the source apparatus is operable to determine a revised change in transmit power that will tend to reduce the said imbalance, and to carry out said revised change.
 16. A communication system as claimed in claim 13, wherein said controller is operable, following issuance of a command to said source apparatus and wherein said instruction was for an increase in transmit power, to monitor said indicator derived by the intermediate apparatus in order to determine if said transmit power of said source apparatus has been changed in accordance with said command.
 17. A communication system as claimed in claim 16, wherein if it is determined that a change in transmit power of the source apparatus has not been carried out in accordance with said command, said controller is operable to prohibit any subsequent increases in transmit power of said intermediate apparatus.
 18. A communication system, as claimed in claim 17, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if no subsequent imbalance is detected by said imbalance detector, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 19. A communication system, as claimed in claim 17, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detector such that said controller of said base station is caused to issue a command to said source apparatus and wherein said command is for a decrease in transmit power, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 20. A communication system, as claimed in claim 17, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detector such that said controller of the base station is caused to issue a command request to said source apparatus wherein said command is for an increase in transmit power which can be carried out by said source apparatus, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 21. A communication system comprising a source apparatus, an intermediate apparatus and a base station, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the wherein each of the base station and the intermediate apparatus comprise: an indicator derivator operable to derive one or more indicators of the quality of a communication signal received at the base station or the intermediate apparatus respectively, the communication system further comprising: i) an imbalance detector operable to detect an imbalance between one said indicator derived by the base station and one said indicator derived intermediate apparatus; and ii) a determination unit operable, following detection of such an imbalance by said imbalance detector, to determine a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance; and iii) a controller provided in said base station and operable, following determination of said change, to issue a command to said source apparatus commanding a change in the transmit power of the source apparatus.
 22. A communication system as claimed in claim 21, wherein one said indicator derived by each of the intermediate apparatus and the base station comprises a measure of the strength of a communication signal respectively received at the intermediate apparatus or the base station, as the case may be.
 23. A communication system as claimed in claim 21, wherein one said indicator derived by each of said intermediate apparatus and said base station comprises a measure of the signal-to-interference plus noise ratio (SINR) of a communication signal respectively received at the intermediate apparatus or the base station, as the case may be.
 24. A communication system as claimed in claim 21, wherein said intermediate apparatus is operable to transmit said indicator to said imbalance detector.
 25. A communication system as claimed in claim 21, wherein said source apparatus is operable, following receipt of a command, and wherein said command is for an increase in transmit power, to determine if it can carry out the command based on the maximum transmit power of the source apparatus.
 26. A communication system as claimed in claim 25, wherein if said source apparatus determines that it cannot carry out the said command, the source apparatus is operable to determine a revised change in transmit power that will tend to reduce the said imbalance, and to carry out said revised change.
 27. A communication system as claimed in claim 24, wherein said controller is operable, following issuance of a command to said source apparatus and wherein said command was for an increase in transmit power, to monitor said indicator derived by the intermediate apparatus in order to determine if said transmit power of said source apparatus has been changed in accordance with said command.
 28. A communication system as claimed in claim 27, wherein if it is determined that a change in transmit power of the source apparatus has not been carried out in accordance with said request, said controller is operable to prohibit any subsequent increases in transmit power of said intermediate apparatus.
 29. A communication system, as claimed in claim 28, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if no subsequent imbalance is detected by said imbalance detector, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 30. A communication system, as claimed in claim 28, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detector such that said controller of said base station is caused to issue a command to said source apparatus and wherein said command is for a decrease in transmit power, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 31. A communication system, as claimed in claim 28, wherein if an increase in transmit power of said intermediate apparatus is prohibited, and if a subsequent imbalance is detected by said imbalance detector such that said controller of said base station is caused to issue a command to said source apparatus and wherein said instruction is for an increase in transmit power which can be carried out by said source apparatus, said controller is operable to allow a subsequent increase in transmit power of said intermediate apparatus.
 32. A communication system as claimed in claim 1 or 13, wherein said intermediate apparatus comprises a regenerative relay node.
 33. A method of controlling the transmit power of an intermediate apparatus in a multi-hop communication system, the communication system comprising a source apparatus, a base station and at least one intermediate apparatus, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the method comprising the steps of: i) deriving, at the base station, one or more indicators of a quality of a communication signal received at the base station; ii) detecting a deviation in the, or one of the, indicators derived by the base station from a desired value; and iii) determining a required change in the transmit power of the intermediate apparatus that will tend to bring said indicator to said desired value.
 34. A method of controlling the transmit power of a source apparatus in a multi-hop communication system, the multi-hop communication system comprising a source apparatus, a base station and at least one intermediate apparatus, the source apparatus being operable to transmit a communication signal, via the or each intermediate apparatus, to the base station, the method comprising the steps of: i) deriving, at each of the base station and the intermediate apparatus, one or more indicator(s) of a quality of a communication signal received at the base station, or at the intermediate apparatus, respectively; ii) detecting an imbalance between one said indicator derived by the base station and one said indicator derived by the intermediate apparatus; iii) determining a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance; and iv) issuing a command to said source apparatus commanding a change in the transmit power of said source apparatus.
 35. A base station operable to receive, via one or more intermediate apparatus a communication signal from a source apparatus, the base station comprising: i) an indicator derivator operable to derive one or more indicators of a quality of a communication signal received at the base station; ii) an indicator deviation detector, operable to detect a deviation in the, or one of the, indicators derived by the indicator derivator; from a desired value; iii) a determination unit operable, following detection of such a change by said indicator deviation detector, to determine a required change in the transmit power of the intermediate apparatus that will tend to bring the said indicator towards said desired value, said determination unit being operable to transmit a request for a change in the transmit power of the intermediate apparatus to said controller; iv) a controller, operable to receive such a request from said determination unit.
 36. A base station as claimed in claim 35, wherein the controller further comprises input signal receiver, operable to receive an input signal which allows the controller to determine if an increase in transmit power of the intermediate apparatus is prohibited.
 37. A base station as claimed in claim 35 or 36, further comprising: i) a receiver, operable to receive an indicator derived by said intermediate apparatus, which indicator is indicative of a quality of a communication signal received at the intermediate apparatus; ii) an imbalance detector operable to detect an imbalance between an indicator derived by the base station and an indicator received from the intermediate apparatus; wherein said determination unit is operable, following detection of such an imbalance by said imbalance detector, to determine a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance, said controller being further operable, following determination of said required change, to issue a command to said source apparatus commanding a change in the transmit power of said source apparatus.
 38. A base station operable to receive, via one or more intermediate apparatus, a communication signal from a source apparatus, the base station comprising: i) an indicator derivator operable to derive one or more indicator of the quality of a communication signal received at the base station; ii) an indicator receiver operable to receive one or more indicator from the intermediate apparatus, wherein the or each indicator is indicative of the quality of a communication signal received at the intermediate apparatus; iii) an imbalance detector operable to detect an imbalance between an indicator derived by the base station and an indicator received from the intermediate apparatus; and iv) a determination unit operable, following detection of such an imbalance by said imbalance detector, to determine a required change in the transmit power of the source apparatus that will tend to reduce such an imbalance; and v) a controller operable, following determination of said change, to issue a command to said source apparatus commanding a change in the transmit power of said source apparatus.
 39. A base station as claimed in claim 38, wherein said controller is operable, following issuance of a command to said source apparatus and wherein said instruction was for an increase in transmit power, to monitor said signal strength indicator derived by the intermediate apparatus in order to determine if said transmit power of said source apparatus has been changed in accordance with said command.
 40. A base station as claimed in claim 39, wherein if it is determined that a change in transmit power of the source apparatus has not been carried out in accordance with said command, said controller is operable to prohibit any subsequent increases in transmit power of said intermediate apparatus.
 41. A computer program which, when loaded into a computer, causes the computer to become the base station of the communication system as claimed in claim
 1. 